Xanthomonas hortorum was recently the subject of renewed interest, as several studies highlighted that its members were responsible for diseases in a wide range of plant species, including crops of agricultural relevance (e.g., tomato and carrot). Among X. hortorum variants, X. hortorum pv. vitians is a reemerging foliar hemibiotrophic phytopathogen responsible for severe outbreaks of bacterial leaf spot of lettuce all around the world.

The essential genome of a bacterium encompasses core genes associated with basic cellular processes and conditionally essential genes dependent upon environmental conditions or the genetic context. Comprehensive knowledge of those gene sets allows for a better understanding of fundamental bacterial biology and offers new perspectives for antimicrobial drug research against detrimental bacteria such as pathogens. We investigated the essential genome of Xanthomonas hortorum pv. vitians, a gammaproteobacterial plant pathogen of lettuce (Lactuca sativa L.) which belongs to the plant-pathogen reservoir genus Xanthomonas and is affiliated to the family Xanthomonadaceae . No practical means of disease control or prevention against this pathogen is currently available, and its molecular biology is virtually unknown. To reach a comprehensive overview of the essential genome of X. hortorum pv. vitians LM16734, we developed a mixed approach combining high-quality full genome sequencing, saturated transposon insertion sequencing (Tn-Seq) in optimal growth conditions, and coupled computational analyses such as comparative genomics, synteny assessment and phylogenomics. Among the 370 essential loci identified by Tn-Seq, a majority was bound to critical cell processes conserved across bacteria. The remaining genes were either related to specific ecological features of Xanthomonas or Xanthomonadaceae species, or acquired through horizontal gene transfer of mobile genetic elements and associated with ancestral parasitic gene behaviour and bacterial defence systems. Our study sheds new light on our usual concepts about gene essentiality and is pioneering in the molecular and genomic study of X. hortorum pv. vitians.

In the face of rising antimicrobial resistance, food insecurity, and climate change, bacteriophages are gaining renewed attention as promising biological alternatives to antibiotics across human, animal, and plant health sectors. Their high specificity, self-replicating capacity, and biodegradability make them valuable tools for combating antimicrobial or pesticide resistance and promoting sustainable biocontrol. This review discusses commonly accepted selection criteria for therapeutic phages, such as avoiding temperate types and screening for undesirable genes, while acknowledging ongoing debates and exceptions in specific clinical or ecological contexts. An overview of phage-based applications within a One Health framework is provided, spanning human medicine, veterinary practice, aquaculture, food safety and crop protection. Particular attention is given to agricultural biocontrol, where several successful plant protection strategies are highlighted, illustrating the feasibility and diversity of phage-based approaches. Despite their potential, key challenges remain regarding phage stability, formulation, and persistence under environmental conditions. Emerging innovations such as encapsulation, carrier bacteria, and protective formulations aim to enhance field performance. Furthermore, this review emphasizes the need to assess the environmental safety of phage applications, particularly their impacts on natural ecosystems, microbial communities, and ecological functions. Finally, the regulatory and policy challenges that currently limit the large-scale deployment of phages, particularly in the European Union, where they remain evaluated under conventional chemical pesticide frameworks are discussed. The development of dedicated regulatory pathways, better adapted to the specificities of phages, is argued to be essential for supporting their integration into agroecological transition strategies and next-generation antimicrobial policies.

The climate crisis presents an urgent challenge for Earth’s living creatures and the habitats in which they have been adapted to thrive. Climate-related stress presents risks to microorganisms, the stability of the functions they provide, and their maintenance of beneficial interactions with their hosts and ecosystems. Microbes move across the continuum of anthropogenic influence on Earth’s ecosystems, from pristine to human-managed to fully urbanized environments. Because microbial feedback within and across this continuum exists at multiple, connected scales from molecules to ecosystem-level processes, predicting microbial responses to climate stress and their potentially wide-ranging consequences remains difficult. Here, we discuss the broad implications of microbial and microbiome responses to climate change as they interface with human, plant, and ecosystem health. For each section on human, plant and ecosystem health, we briefly discuss the state of knowledge for each and follow with proposed future research, including some directions that are promising but require more work to evaluate. We end by considering overarching microbial ecology research needs across these systems and microbial solutions under investigation as possible climate-resilient interventions to maintain human, plant, and ecosystem health. This work draws on diverse expertise to identify broad research directions across typically separated disciplines and builds a holistic framework for considering their interrelationships.

Due to limitations in disease management strategies and the impact of climate change, phytopathogenic bacteria are threatening global crop production. Xanthomonas hortorum pv. vitians, the causal agent of bacterial leaf spot of lettuce, remains difficult to manage due to the lack of efficient treatment options. As an alternative, phage‐based biocontrol offers a promising solution, but its long‐term efficacy depends on a thorough understanding of phage–host interactions and the potential development of bacterial resistance. In this study, we isolated and characterised the lytic bacteriophage Φ Xhv ‐1, which defines a novel genus within the class Caudoviricetes . Using transposon insertion sequencing, we identified 36 bacterial genes essential for phage susceptibility, primarily involved in lipopolysaccharide biosynthesis and surface polysaccharide modifications. Targeted mutagenesis and fluorescence microscopy confirmed that Φ Xhv ‐1 adsorbs onto specific residues of the LPS O‐antigen side chains. Phage‐resistant mutants exhibited a decreased motility in vitro and a significant reduction in virulence in planta. These findings reveal a strong evolutionary trade‐off between phage resistance and bacterial fitness, suggesting that resistance emergence in the field may be naturally constrained. This study provides new insights into X. hortorum pv. vitians –phage interactions and supports the development of sustainable phage‐based biocontrol strategies against bacterial leaf spot of lettuce.

In recent years, the use of bacteriophages as biocontrol agents has gained renewed attention, especially for targeting major phytopathogenic genera like Xanthomonas . However, their potential against Xanthomonas hortorum pv. vitians , the causal agent of bacterial leaf spot of lettuce, remains largely unexplored, with only two virulent phages reported to date. In this study, eight novel virulent tailed phages were isolated from wastewater samples, including three jumbophages, which are the first known to infect this pathovar. These jumbophages define a new viral genus and exhibit distinct biological characteristics, including broader host ranges, but lower burst sizes and slower lytic cycles compared with the five smaller-genome phages. Together, the newly isolated phages exhibited lytic activity against nearly all X. hortorum pv. vitians strains tested while remaining highly specific to the pathovar. Genomic analyses revealed strictly lytic profiles, and absence of virulence or resistance genes, supporting their biosafety for agricultural use. Furthermore, the isolation of a phage infecting a bacterial mutant previously shown to be resistant to the virulent phage Phi Xhv -1 highlights the potential for designing robust phage cocktails to mitigate resistance emergence. All phages demonstrated high stability across a wide range of pH and temperature conditions but remained highly sensitive to UV exposure. These findings provide a valuable foundation for developing phage-based biocontrol strategies against bacterial leaf spot of lettuce. IMPORTANCE: Ranked among the leading plant pathogenic bacterial genera, Xanthomonas species affect a wide range of crops and contribute to major agricultural losses worldwide. Among these, Xanthomonas hortorum pv. vitians , the causal agent of bacterial leaf spot of lettuce, poses a serious threat to lettuce production worldwide. Currently, no effective disease managements strategies are available to reduce this foliar hemibiotropic phytopathogen. Bacteriophage-based biocontrol has shown promising results against Xanthomonas spp. However, its application against this pathogen remains largely unexplored, with only two phages isolated to date. Successful phage application in agriculture requires careful selection and through characterization to ensure safety and efficacy. In this study, the phage diversity targeting this phytopathogen was expanded through the isolation of eight novel virulent phages, including the first jumbophages infecting this species. Their genomic and biological properties provide a valuable foundation for developing future phage-based treatment against this challenging pathogen.